1. Field
This invention is directed to coatings for drug delivery devices, such as drug eluting vascular stents, and methods for producing the same.
2. Description of the State of the Art
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease, which often manifests itself as stenoses in coronary arteries due to atherosclerosis. A surgeon inserts a catheter assembly having a balloon portion through the skin into a patient's cardiovascular system by way of the brachial or femoral artery. The surgeon positions the catheter assembly across the occlusive lesion. The surgeon inflates the balloon, once positioned, to a predetermined size to radially compress the atherosclerotic plaque of the lesion and to remodel the artery wall. After deflating the balloon, the surgeon withdraws the catheter from the patient's vasculature.
But sometimes this procedure forms intimal flaps or tears arterial linings. These injuries can collapse or occlude the vessel. Moreover, the artery may develop thrombosis and restenosis up to several months after the procedure and may require further angioplasty or a surgical by-pass operation. Implanting a stent into the artery can rectify the injuries and help preserve vascular patency.
In a related manner, local administration of therapeutic agents with stents or stent coatings has reduced restenosis. But even with the progress in stent technology in recent years, stents still can cause undesirable effects. For example, the continued exposure of a stent to blood can lead to thrombus formation itself, and the presence of a stent in a blood vessel can weaken the blood vessel wall over time, which may allow arterial rupture or the formation of an aneurism. A stent can also become so tissue overgrown that it becomes less useful and that its continued presence may cause a variety of problems or complications. Therefore, biodegradable or bioabsorbable stents are desirable to diminish risks that would otherwise associate with the continued presence of a no-longer-needed device at the treatment site.
Polymeric stent coatings can cause adverse and inflammatory reactions in vivo. And there is much less history of using polymerically coated stents, while bare metal stents have an extensive history. Use of absorbable or resorbable coatings also allows for drug release profiles that are difficult to achieve with non-absorbable polymers. Hence, there is great interest in using erodable, absorbable, or resorbable coatings on stents. Next, device coatings with non-fouling properties are desirable. Non-fouling compounds such as poly(ethylene glycol) (PEG) provide these properties. But in order for a copolymer containing PEG to possess non-fouling properties, it is believed that the copolymer must present a high concentration of PEG at the polymer-water interface—to repel protein because repelling proteins requires this. High PEG concentration in the copolymer can deleteriously affect other coating performance aspects. For example, high PEG levels can significantly increase water swelling. This, in turn, can lead to too rapid drug release. It can also reduce the coating's mechanical properties, compromising its durability. Accordingly, there is a need for non-fouling coatings based on biologically absorbable or biologically degradable polymers that are simultaneously non-fouling and that have the drug release and mechanical properties suitable for a coating.